This invention relates to an antilock control system for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon a variety of parameters including the road surface conditions and the amount of slip present between the wheel and the road surface. This braking force increases as slip increases, until a critical value of slip is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock control system seeks to operate wheel slip at or near the critical slip value. An antilock control system achieves this by detecting an incipient lock condition. Upon detecting an incipient lock condition, the antilock control system releases pressure at the wheel brake. Once the wheel recovers from the incipient lock condition, brake pressure can be re-established. An incipient lock condition is typically represented by the magnitude of wheel deceleration exceeding a deceleration threshold value or slip exceeding a slip threshold value.
However, certain characteristics of the vehicle suspension and/or the road surface can cause the wheel to exhibit behavior as if it is approaching an incipient lock when in reality it is not. For example, on deformable road surfaces, the brake force-wheel slip relationship does not follow the relationship previously described where a critical slip is reached after which braking force decreases with increasing slip but instead follows the relationship wherein brake force continuously increases with increasing slip. In these cases, the antilock system takes corrective action (requesting a release of brake pressure) based upon the prediction that an incipient lock condition will soon exist. This release of pressure will then typically cause the wheel to suddenly exhibit a recovery condition. However, this exhibited recovery condition is short-lived, as the reapplication of pressure usually results in the prediction of an incipient lock condition and release of brake pressure in response thereto. Thus, a wheel recovery following a sensed incipient lock when an incipient lock condition does not in fact exist occurs much sooner than would a wheel recovery following a true incipient lock condition, and relatedly, an incipient lock condition following a recovery from a detected incipient lock when an incipient lock condition does not in fact exist also occurs much sooner than it would under normal conditions. This results in a rapid cycling behavior of the antilock system.